Space research in Earth orbit, lunar, and interplanetary environments has resulted in an urgent demand for autonomous observing technologies in recent years. One technique for autonomous observation is based on active imaging sensors. Active 3D imaging systems are attractive for a wide range of applications, such as surface reconstruction, mapping, landing of probes, obstacle recognition and navigation for vehicles, rendezvous, and docking maneuvers.
Much effort has been made to develop a 3D imaging laser radar based on a time-of-flight (TOF) technique. In this technique, a very short laser pulse is sent out towards the object and the scattered light from the object is collected. The time delay between the start pulse and the returned pulse is measured to determine the object distance. With this technique, a high peak power laser is required. In many cases, a relatively complicated diode pump, such as a passively Q-switched solid-state laser, is used as a light source.
However, applications of space-based lidars that require compact size, light weight, and reliability are usually constrained by the laser source. To date, the smallest devices appropriate for such applications are diode lasers. Unfortunately, compact semiconductor lasers have peak power levels well below the requirements of lidar systems based on TOF techniques. Thus, a pseudo-noise (PN) coding technique for 3D imaging may be beneficial.
PN code may include a spectrum similar to a random sequence of bits but is deterministically generated. PN code modulation may be widely used in RF communications. It may typically be modulated in non-return-to-zero (NRZ) format, where the transmitted pulse width equals to the coding bit period. Timing measurement accuracy using PN code technique may be proportional to the transmitted pulse width divided by the measurement signal to noise ratio. Shorter transmitted pulses result higher measurement accuracy for the same transmitted average power. Traditionally, PN code using NRZ format modulates the signal at 50% duty cycle. PN code modulation using an advanced scheme with a lower duty cycle may substantially increase the measurement accuracy.